Use GFM Markdown fixtures in Content Editor

What does this MR do?

Context: We generate frontend fixtures to test the compatibility of the Content Editor with GitLab Flavored Markdown. These fixtures are generated by sending small snippets of Markdown to the Markdown REST API. See the description of !55856 (merged) for more information.

This MR passes the gfm: true parameter to the Markdown API call to process the Markdown snippets using GitLab Markdown extensions. In cases like processing images or code blocks, the API output completely changes. As a result, this MR also introduces fixes to the image and code_block extensions to properly process the API output.

How to test?

• Generate the Markdown API fixtures: bundle exec rspec spec/frontend/fixtures/api_markdown.rb
• Run the jest tests: yarn jest --watch spec/frontend/content_editor/markdown_processing_spec.js

If you are interested in understand how the extensions are parsing the API output, check the API output in tmp/tests/frontend/fixtures-ee/api/markdown/

Screenshots (strongly suggested)

This change is behind a feature flag and it doesn’t provide user-facing changes.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team